Photolithographically patterned stress-engineered metal structures (e.g., spring probes) have been developed, for example, to produce low cost probe cards and to provide electrical connections between integrated circuits. A typical spring structure is formed from a stress-engineered (a.k.a. “stressy”) metal film intentionally fabricated such that its lower/upper portions have a higher internal tensile stress than its upper/lower portions. The internal stress gradient is produced in the stress-engineered metal film by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. The stress-engineered metal film is patterned to form islands that are secured to an underlying substrate either directly or using an intermediate release material layer. When the release material (and/or underlying substrate) is selectively etched from beneath a first (free) portion, the free portion bends away from the substrate to produce a spring structure that remains secured to the substrate by an anchor portion. Such spring structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. Examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
When used to form probe cards, such spring metal structures must exhibit sufficient stiffness to facilitate proper electrical connection between the probe (spring metal finger) and a corresponding contact pad on the device-under-test. Most stressy metal spring probes produced by conventional methods are fabricated from sputtered metal that is approximately one micron thick, which produces only a nominal stiffness capable of resisting a force of 0.1 to 0.2 grams (gmf). These stressy metal spring probes may provide sufficient stiffness to probe gold contact pads, but are not stiff enough to reliably probe aluminum pads. Gold pads can be readily probed with relatively weak spring probes because gold does not form a passivation layer that takes significant force to puncture. However, aluminum pads form a passivation layer that must be punctured by the tip of the spring probe in order to facilitate proper electrical connection. To repeatedly achieve electrical contact to aluminum, which is required for many integrated circuit probe card applications, deflection of the probes within their elastic region should absorb an expected force of several grams.
One method of increasing the stiffness of stressy metal spring probes is to increase the probe thickness. Using a linear spring model, the force (F) imparted by a stressy metal spring probe is roughly equal to the product of the vertically compressed deflection from the relaxed state (dZ) multiplied by its vertical stiffness (K). Sputtered stressy metal probes have been produced that achieve large vertical deflections (up to approximately 100 microns), but their corresponding vertical stiffness is limited by the relatively thin sputtered metal. One approach to increasing the probe thickness, and hence the probe stiffness, is to produce thicker stressy metal films. However, fabricating a spring probe entirely from stressy metal would require sputtering tens of microns of metal, which would be very time consuming and hence very expensive. A less expensive approach to generating a higher force is sputtering and releasing a relatively thin (and therefore less expensive) stressy metal probe structure, and then thickening the probe using a relatively inexpensive plating process. Plating a thick metal layer (a few microns) on the probe significantly increases probe stiffness, but could also decrease the maximum deflection (dZ). Maximum deflection is determined by the initial lift height and the fracture limit of the probe. Laboratory experiments have shown thick electroplated stiffened springs break or yield when deflected a significant fraction of their initial lift height. Soft plated metals such as gold break less readily than hard plated metals such as nickel. Failure typically occurs at the base (anchor portion) of the cantilevers, where plating forms a wedge that acts as a stress-concentrating fulcrum to pry the base away from the underlying substrate as the probe is deflected. The wedge forms because the thickness of the release layer formed under the stressy metal film (if used) is less than the thickness of the plated layer. This wedge limits the maximum force because it limits both the allowed thickness of the plating (K) and the maximum displacement (dZmax).
Accordingly, what is needed is a cost effective method for fabricating high force spring probes and other spring structures from stress-engineered metal films that are thick (stiff) enough to support, for example, large probing forces, but avoid the production of a wedge at the base (anchor portion) of the spring that has a tendency to pry the spring base off the substrate surface.